Floating offshore structures, which are used for drilling or production while being floated on the sea, demonstrate movements, such as rolling, pitching and heaving, by waves, winds and tides. Accordingly, it is important to minimize these movements in order to maximize the efficiency of a floating drilling/production facility.
Proposed recently as a floating structure for production are a structure such as a spar or a buoy, whose height is substantially greater than its diameter, and a structure proposed by SEVAN that has a substantially greater diameter than its height. These structures have various shapes, including cylindrical shapes, rectangular shapes and octagonal shapes, and aim to achieve stability through a center of mass that is lower than a center of buoyancy of the submerged structure.
Unlike a ship, the floating offshore structures such as the spar and the buoy, which have a substantially greater height than the diameter, are designed with an ideal shape having a small water plane area in order to minimize the rolling, pitching and heaving. However, these offshore structures have an elongated shape, which is difficult to make, transport and install, and cannot include a storage function.
In the meantime, in order to complement the spar or the buoy with the storage function, a cylinder-shaped floating offshore structure having a greater diameter than its height (hereinafter, “SEVAN-type offshore structure”) is proposed. As the SEVAN-type offshore structure has the shape of a cylinder, rolling and pitching are dramatically reduced.
However, in terms of dealing with heaving of the SEVAN-type offshore structure, the diameter of the cylindrical structure becomes greater as the storage capacity increases, resulting in the increase in the water plane area.
Accordingly, the natural period of heaving of the SEVAN-type offshore structure becomes shorter and demonstrates a tendency to be close to a wave period in an extreme wave condition with a repetition period of 100 or more years that is generated by a typhoon or abnormal weather. When the natural period of the SEVAN-type offshore structure becomes close to the wave period, a phenomenon of resonance occurs, causing an excessive heaving movement.
Moreover, in order to prevent such an excessive heaving movement, an excessive mooring system is required to stabilize the SEVAN-type offshore structure, but the SEVAN-type offshore structure becomes inoperable if the heaving movement exceeds the designed value of the mooring system.
In the meantime, the conventional ship-type of offshore structure includes a plurality of cargo tanks and ballast tanks for storing the produced resources. In such a case, each tank is installed with a submerged pump. Not only is the submerged pump an expensive equipment, but an excessive costs are required because each tanks needs to be equipped with one submerged pump.